The present invention relates to an integrated device for shielding charge injection into the substrate, in particular in driving circuits for inductive and capacitive loads.
As is known, during the driving of inductive and/or capacitive loads, in order to achieve a correct behaviour of the system it is necessary to discharge the current accumulated at the load towards the ground or towards the supply. When load driving is performed by means of integrated devices, the substrate of the device is generally clamped to the ground and the current is discharged through the diode formed by the junction of an epitaxial pocket connected to the load (which pocket defines the cathode) and substrate (which defines the anode of the diode). In order to allow flyback of the current upon switching off of the load driving devices, this diode is directly biased and the epitaxial pocket reaches a negative voltage with respect to the ground.
In this condition, the epitaxial pocket with negative voltage injects current into the substrate, behaving like the emitter of a parasitic NPN transistor, and every other epitaxial pocket of the same device which is at a greater voltage with respect to the substrate collects the injected electrons behaving like the collector of the parasitic transistor having the substrate for base and the negative epitaxial pocket for emitter.
This injection of charges in epitaxial pockets which are at high voltage entails some problems and therefore must be avoided. In fact, in particular, it causes an undesired dissipation of power; if the epitaxial pocket which is at high voltage is the base of a PNP transistor, it causes switching-on of said lateral PNPs (including, for example, the driving ones); it furthermore causes an increase in the base current of the lateral PNP transistors, thus causing a malfunction in the device.
In order to avoid the above described problems, two solutions are currently used, both having the purpose of creating a path which has lower impedance than the other epitaxial pockets of the circuit and therefore such as to collect all the current injected by the epitaxial pocket which is at a negative voltage with respect to the ground. Such solutions have a similar structure which comprises an isolated epitaxial pocket which surrounds the entire epitaxial pocket which can reach a negative voltage with respect to the ground. Said isolated pocket comprises, in its interior, a buried layer, as well as a sinker layer, which are more heavily doped than the epitaxial layer. This structure is illustrated for the sake of clarity in FIG. 1 and can be connected to the ground or to the power supply.
Though these solutions partially improve the problem, they are however still unsatisfactory since they are unable to completely eliminate the current towards other active regions of the circuit, and they do not reduce the dissipated power. Moreover, in particular, the solution comprising the connection to the ground is scarcely competitive with respect to high-voltage epitaxial pockets which tend to provide a better collecting of the charges injected by the negative voltage epitaxial pocket, thus producing results which are not fully satisfactory. On the contrary, the solution wherein the described structure is connected to the power supply has a better behavior as to collection of the injected current, but it is disadvantageous in terms of power dissipation since the current of the parasitic transistor is collected towards the point at the highest voltage.